According to the modern trend in the automobile industry, the various vehicle accessories, such as power steering pump, oil and air pumps, air conditioning and alternator, are operated by a single endless belt driven by a pulley connected to the engine crankshaft. Such system is referred to as a "serpentine" drive belt system. To provide optimum operating efficiency for the above-mentioned and other various accessories, it is imperative that a predetermined tensioning force of the drive belt be maintained to assure efficient performance of the accessories as well as satisfactory service life for the belt. As a result of the relatively greater length of the single drive belt which replaces the heretofore plurality of smaller belts, the belt tends to stretch. Such stretching affects the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device to be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereon regardless of the amount of belt stretch. Hence, it is conventional practice in the belt tensioner art to provide for the application of a constant belt tensioning force which compensates for increases in belt length due to wear and vibration.
Countless attempts have been made to accomplish this purpose. A common type of conventional belt tensioner embodies a stationary housing and an angularly displaceable lever arm that carries a belt engaging pulley. A coil spring is braced against the stationary housing and displaceable lever arm and biases the latter toward the belt with a tensioning force varying in accordance with the vibrational nature of the belt. Despite this varying spring force a substantially constant force acting upon the lever arm is maintained.
For example, a belt tensioning device has been proposed in U.S. Pat. No. 3,924,483. This patent discloses a torsional spring for pivotally moving one of the vehicle accessories to achieve the desired tensioning force. Other tensioners of the above-described type utilize a pair of torsional springs for pivotally moving a lever and an idler pulley into belt tensioning engagement which results in an economic and compact unit. Specifically, in this type of tensioner, each spring is mounted on a respective side of the lever and engaged with the lever and housing for biasing the intervening lever toward the belt in a belt tensioning direction. Furthermore, the automobile industry has recognized the vibrational environment of an automobile belt system and its effect on spring oscillation.
It is desirable that a belt tensioner be provided with a damping means to prevent excessive oscillation from occurring in the spring member. Such means is designed to absorb sudden shocks and to prevent a whipping action from occurring in the tensioner and drive belt. This damping means is especially critical when a coil spring is used for applying the belt tensioning force since it is inherent to coil springs to develop natural oscillation frequencies upon applying of the fluctuating counter force by the belt. Such fluctuations diminish the efficiency of tensioning force of the spring. However, the damping requirements are essential in order to enable the belt system to function over an extended period on a pulsating machine without affecting a tensioning force that acts upon the drive belt.
U.S. Pat. No. 4,696,663 discloses a belt tensioner that includes a stationary housing 12, a lever arm 30, and a torsional spring 20 which is braced against the housing and the lever and biases the lever in a belt-tensioning direction. The belt tensioner is equipped with a brake 60 actuated by the spring into frictional engagement with a housing wall 13. Since the torsional spring provides both the tensioning force for the lever and the actuating brake force, the amount of damping is proportional to the belt tensioning force.
U.S. Pat. No. 4,473,362 discloses a separate damping body 108 whose damping characteristics are not constant but vary proportionately with the position of a pivot structure 40 relative to a stationary structure 36. A coil spring is mounted between the fixed and pivoted structures for resiliently biasing the latter in a direction away from the first limiting position thereof toward the second limiting position with a spring force, which increases as the pivot structure is displaced toward the belt. The damping body has a relatively tight fit at its inner periphery with the outer periphery of a core member 48 and a relatively loose fit between its exterior periphery and an interior periphery of the pivot structure. Angular displacement of the pivot structure between its first and second limiting positions is accompanied by a sliding movement between the exterior periphery of the damping body and the inner periphery of a mounting portion of the pivot structure. Since the radial pressure between these two contacting surfaces varies in accordance with the position of the pivot structure, the amount of friction likely vary as well and hence the torsional force required to overcome the frictional force may also vary. Thus, the arm advantageously experiences a greater damping effect in a belt-releasing direction.
A basic structure of a known prior art tensioner is shown in FIG. 1 and includes a stationary housing 3', a lever arm 4' operatively connected with the housing, and a spring 30' braced against the stationary housing and lever arm for generating the spring tensioning force. The stationary housing is mounted on an engine bracket adjacent to the belt system and, preferably, is formed of sheet metal or similar rigid material. Mounting of the housing is provided by a shaft 2'. The lever arm is pivotally mounted on the shaft and is formed with a radially outwardly extending raised arm, which terminates in a distant arm end. An idler pulley is operatively connected to the arm end for synchronous pivotal movement therewith toward and away from the belt system that is to be tensioned. A torsional spring is connected to the housing and the lever arm in a known manner and provides a tensioning force making the arm biased in a belt-tensioning direction. The lever arm is formed with a compartment 25' receiving a dampening device 11' which generates a uniform force tending to slow down the lever arm regardless of the direction in which the arm pivots.
It is desirable to have a structure that allows the lever arm to swing in a belt-tensioning direction relatively freely while, during rotation of the lever arm in a belt-releasing direction, to have a substantial force applied to the lever arm so as to provide high damping thereon.